It is known to carry out the drying of ultrafiltration membranes after impregnation with glycerin. However, both in connection with cellulose acetate membranes and also in connection with cellulose hydrate membranes, there occurs a reduction of the flow-through output, and in hydrate membranes there arises the additional difficulty that a disturbance-free dryability is not assured, and also the protein separation is not retained.
While it is true that there are available cellulose acetate membranes with excellent properties, in some cases, for example when the membrane to be employed must be resistant to solvents, cellulose acetate membranes and specifically also cellulose triacetate membranes and most of the other polymeric membranes cannot be used because they are destroyed by swelling or dissolution. Cellulose hydrate, on the other hand, is insoluble in organic solvents and may therefore be employed without difficulty.
It is known that asymmetrical cellulose acetate membranes may be converted by saponification into cellulose hydrate membranes having the same structure (German Offenlegungsschrift No. 2,419,930). The saponification is, however, always accompanied by a more or less pronounced deterioration of the membrane properties (decrease of the flow-through rate and of the protein density). This may be of considerable disadvantageous significance also for acetate membranes, and particularly in a case where the saponification occurs unintentionally during the operation, i.e. as a result of extreme pH values.
It is known to carry out the drying of ultrafiltration membranes, particularly acetate membranes, after impregnation with glycerin, wherein the impregnation is intended to prevent an irreversible flow-through reduction and embrittlement or shrinkage. However, the drying of asymmetrical cellulose hydrate ultrafiltration membranes, because of their high water absorption capacity, is accompanied, even after glycerin impregnation, by a further strong output reduction and/or destruction by shrinking.
Known cellulose hydrate ultrafiltration membranes must therefore be stored and treated in the moist condition, which renders processing thereof difficult. Furthermore, it is necessary in this case to add preservation agents during storage because cellulose hydrate when in the moist condition represents a nutrient medium for germs (bacteria, fungi).